Lubricating oils for medium- or low-speed diesel engines typically contain a range of additives which perform a variety of functions: for example they may comprise dispersants to minimise deposit formation in various parts of the engine or detergent additives. Contamination of these lubricating oil compositions with unburnt residual fuel oil is a problem recognised in the industry. This leads to severe engine cleanliness problems in service which is sometimes referred to as "black paint." The problem is particularly widespread in 4-stroke trunk-piston engines where dirty cam boxes and crankcases are encountered. However, the problem is not confined to 4-stroke engines; 2-stroke cross-head engines may also suffer from the problem. These 2-stroke engines typically use two separate lubricating oils, one for the crankcase and one for the cylinder, but it is in the crankcase where the heavy deposits typically occur. It might be expected that the problem would be overcome simply by using more of the conventional dispersant additive in the lubricating oil, but this measure has met with very limited success.
Acidity in lubricating oil is another long-recognised problem. In the operation of the internal combustion engine by-products from the combustion chamber often blow by the piston and admix with the lubricating oil. Additives are generally employed to neutralise the acidic materials and disperse sludge within the lubricating oil. Examples are overbased alkaline earth metal sulphurised hydrocarbyl-substituted phenates, salicylates, napthenates and sulphonates. Overbased calixarates are also known as detergent additives for lubricating oils, eg EP-A-450874. The term "overbased" is generally used to describe those metal salts in which the ratio of the number of equivalents of the metal moiety to the number of equivalents of the acid moiety is greater than one, and is usually greater than 1.2 and may be as high as 4.5 or greater. In contrast, the equivalent ratio of a metal moiety to acid moiety in "normal" or "neutral" metal salts is one, and in "low-based" salts is less than one. Thus, the overbased material usually contains greater than 20% in excess of the metal present in the corresponding neutral material. For this reason overbased alkaline earth metal hydrocarbyl-substituted salts have a greater capability for neutralising acidic matter than do the corresponding neutral alkaline earth metal hydrocarbyl-substituted salts, though not necessarily an increased detergency power. The degree of overbasing is expressed as "Total Base Number" or TBN, which is also sometimes referred to as Alkalinity Value or AV, and is measured by the method of ASTM D2896.
EP-A-708171 discloses linear molecules comprising optionally substituted phenol units linked by alkylene bridges, with one end of the chain similarly linked to a salicylate moiety. The number of aromatic units thus linked is said generally not to exceed 4, or preferably 3. Overbased metal salts of these compounds are disclosed as being useful as detergents or dispersants, particularly in respect of asphaltene compounds, which are responsible for black paint.
With the present invention the problem of black paint is substantially reduced or eliminated by including in the lubricating oil novel calixarene compounds which contain within the calixarene ring at least one salicylic acid unit. Overbased metal salts of these compounds also function as high TBN detergents, thereby providing two functions in one product. Furthermore, the performance of conventional gasoline and diesel detergents/dispersants is enhanced by combining them with such compounds.